March 20, 2019
Dietary Advice Goes Global

Dietary Advice Goes Global

by Berkeley Wellness  

In August 2017, a large multinational study made headlines when it linked a high carbohydrate intake with higher mortality rates, and a high fat intake with lower rates, seemingly overturning conventional dietary wisdom. What has it taught us, if anything?

The PURE (Prospective Urban Rural Epidemiology) study included 135,000 people from 18 countries: three high-income (Canada, Sweden, and United Arab Emirates), 11 middle-income (such as Brazil, Malaysia, and Iran), and four low-income (Bangladesh, India, Pakistan, and Zimbabwe). Participants filled out a dietary questionnaire and were followed for an average of 7½ years.

Among the main findings, published in the Lancet:

  • Diets highest in carbohydrates (averaging about 77 percent of daily calories, which is very high) were associated with a 28 percent higher death rate than those lowest in carbs (46 percent of calories).
  • Diets highest in fats (averaging 35 percent of daily calories, which is not very high) were associated with a 20 percent lower death rate than those low in fats (less than 20 percent of calories). Very low intakes of all types of fats, including saturated fats, were linked to higher death rates; polyunsaturated fats appeared to be most beneficial.
  • Replacing carbs with polyunsaturated fats (5 percent of daily calorie intake) would reduce mortality rates by 11 percent, the study estimated. Replacing carbs with saturated or monounsaturated fat or protein would have no such effect.
  • Surprisingly, death rates from cardiovascular disease were not affected by variations in fat or carb intake. Only “noncardiovascular mortality,” which includes cancer and infectious and respiratory diseases, was affected.

Casting too wide a net?

While the study’s findings may have important global implications, it’s unclear how much they apply to people in high-income countries, who made up only 11 percent of the participants. Pooling data from such disparate regions may be problematic.

In low-income countries, such as in South Asia, a very high intake of carbs (usually from white rice, white bread, sugar, and starchy vegetables) and very low intake of animal products (main sources of saturated fats as well as many key nutrients) are typically associated with poverty, which in turn is associated with poorer health. Most poor people there do not choose to eat this way; it’s all they can afford, and it leaves them prone to nutritional shortfalls.

The researchers adjusted the data for factors such as age, smoking, education, and energy intake but not for household income, so poverty almost certainly influenced the findings.

Another thing that’s unclear: The data did not discriminate between “bad” carbs (as in refined grains and sweets) and “good” carbs (as in fruits, vegetables, and whole grains). Refined carbs predominate in the highest-carb diets around the world, and it’s still likely that “good” carbs are indeed good for health and longevity.

In fact, a second PURE analysis in the same issue of the Lancet found that fruits, legumes, and vegetables are associated with lower mortality rates; it did not look at different types of grains.

Bottom line

The government’s Dietary Guidelines for Americans 2015-2020 have already moved away from advising a cap on total fat intake, though they do recommend limiting (not avoiding) foods high in saturated fats. And they now recognize that replacing foods high in saturated fats with those high in refined carbs is a recipe for poor health. Instead, they advise focusing on healthy sources of fat such as fish, nuts, and vegetable oils.

They also advise consuming more whole grains and fewer refined carbs and sugars. The PURE findings do not necessarily conflict with that general advice, except they suggest that less attention needs to be paid to reducing intake of saturated fats and more to cutting down on excessive carb intake. They may well be relevant in poorer countries and in poorer parts of richer countries, where under-nutrition is common and low-quality carbs may provide more than three-quarters of daily calories, but their applicability to most people in richer countries is best viewed with skepticism.

Also see The End of the Debate? Fat Chance for more on the latest thinking about saturated fat.